In recent years, a neutron capture therapy for selectively exterminating cancer cells has been studied and put into clinical use in reactor facilities. Such a therapy requires the use of neutrons (thermal neutrons and epithermal neutrons) and hence had no choice but to be performed in the reactor facilities. In spite of the fact that the neutron capture therapy is clinically and medically recognized to have an effect on malignant cancer, the neutron capture therapy has not become widely used. Nuclear reactors are not originally medical facilities, and hence the nuclear reactors are not targeted for advanced medical technology. As a result, there arises a problem of radiation exposure of involved parties, and it is very inconvenient to transport a patient to a nuclear reactor facility.
In the foregoing situation, a neutron generating apparatus that may obtain neutrons without using a nuclear reactor has been studied worldwide. Specifically, the above-mentioned various problems may be solved by a neutron generating apparatus using an accelerator for accelerating charged particles such as electrons, protons, and ions at high speed. The neutron capture therapy is a cancer treatment method involving preparing an agent of a substance which is likely to effect a nuclear reaction with thermal neutrons or the like, for example, a compound containing boron-10 (B-10) which is a nonradioactive isotope, administering the prepared agent to a human body in advance so that the agent is taken into only a region in which cancer is present, that is, only cancer cells present among normal cells, and irradiating a cancer site with neutrons (thermal neutrons and epithermal neutrons) which have less effect on human bodies, to thereby selectively exterminate only the cancer cells. In this case, it is important how boron-10 is selectively taken into the cancer cells. Thus, the neutron capture therapy is a treatment method of exterminating only cancer cells through a nuclear reaction between the boron compound (for example, boronophenylalanine) and the neutrons. This method is called a boron neutron capture therapy (BNCT). In order to realize the neutron capture therapy, cutting-edge technologies in multiple fields are required (see, for example, Non-Patent Literature 1). This document relates to a “target unit” serving as a main technology, that is, a “core technology” of a neutron generating apparatus using an accelerator for realizing the neutron capture therapy.
More specifically, the above-mentioned neutron capture therapy uses a phenomenon that a certain kind of a boron compound which accumulates only on cancer cells efficiently reacts with neutrons having low energy such as thermal neutrons and epithermal neutrons. When a boron compound, for example, boronophenylalanine is infused into a patient, boronophenylalanine accumulates on cancer cells at a very high rate. When a site in which cancer is present of the patient having received boronophenylalanine by infusion is irradiated with thermal neutrons and epidermal neutrons, boron-10 of boronophenylalanine and neutrons react with each other at a cell level to release an alpha-ray having high energy of 2.33 MeV with a strong cell-killing effect, with the result that only the cancer cells are damaged by the alpha-ray to be exterminated. The range of the alpha-ray is less than 10 microns, and hence the boron neutron capture therapy (BNCT) may be considered as a cutting-edge treatment method that may distinguish normal cells from cancer cells and exterminating only the cancer cells.
A neutron source using an accelerator includes, for example, an accelerator for accelerating protons and a target unit disposed on a downstream side of the accelerator. The accelerator includes an ion source, an LEBT, an RFQ linac, an RFI linac, and the like. The target unit contains a lithium target, and protons accelerated by the accelerator are caused to collide with the lithium target to generate neutrons through a nuclear reaction (see, for example, Patent Literature 1).